1. Technical Field
This invention relates to systems for controlling the current provided to an alternator of a motor vehicle, and more specifically to a method for controlling the duty cycle of a pulse-width-modulated signal applied to an alternator field winding, which provides a high degree of control over the resolution of the duty cycle to thereby afford a high degree of control over the signal applied to the field winding of the alternator.
2. Discussion
The basic method of regulating the voltage level of a battery of a motor vehicle during vehicle operation consists of controlling the RMS electrical current supplied to the field winding of the vehicle's alternator. The amount of energy that the alternator produces is directly proportional to the RMS electrical current being supplied to the field winding of the alternator and is also directly proportional to the speed at which the alternator armature is rotating. Controlling the RMS electrical current being supplied to the alternator field winding is typically achieved by delivering a pulse-width-modulated control signal to the alternator field circuit. In the steady-state mode, this can be accomplished by driving the alternator field control output port of a microprocessor associated with an engine controller of the vehicle in a prescribed "ON"/"OFF" pattern that effectively delivers one of several different duty cycles for the pulse-width-modulated (PWM) signal. For example, seven different duty cycles (i.e., 0%, 12.5%, 25%, 50%, 75%, 87.5% and 100%) could be employed. The duty cycle value delivered to the alternator field circuit could be updated once every few milliseconds (such as 3 ms-4 ms). The decision as to which one of the seven duty cycles to use could simply be based on the magnitude and sign of the difference between the most recent battery voltage level and the voltage regulator set-point (the "desired" or "goal" battery voltage value). The measure of the battery voltage level could be obtained from an analog to digital conversion that would occur in the duty cycle control routine.
While the above-described method of choosing a duty cycle is adequately effective for applying several different, predetermined duty cycles to the alternator to control the percent of time that the field winding of the alternator is energized, the method does not consider the electrical load level on the engine of the vehicle at any given time, nor does it consider the rotational speed of the engine at any given time. Furthermore, the limited choices of duty cycles make it difficult to accurately control the battery voltage level over the wide range of engine operational conditions. All these factors impact on the system performance and contribute to a relatively "noisy" signal being applied to the alternator field winding which results in a similar "noisy" signal being applied to the battery (and the whole system) from the alternator output.
Possibly the biggest drawback of the present method of applying one of several predetermined duty cycles is that this method is limited in managing the application of higher alternator field duty cycles in response to "step" or "impulse" electrical loads. Such impulse electrical loads may be generated by activation of such devices as the radiator fan motor, the rear window heater and/or window motors typically employed in motor vehicles, just to name a few. This limitation can significantly contribute to engine speed instabilities at or near park/neutral and drive/reverse idle speeds. Presently used algorithms typically employ a few different methods to detect these types of loads and then attempt to manage the application of increasing alternator duty cycles. For example, when it is desired to turn the radiator fan motor "ON," one method first opens the idle speed bypass valve a prescribed number of steps, delays a prescribed length of time and then turns the radiator fan motor "ON." Unfortunately, this method typically causes the engine speed to flare slightly before dropping noticeably when the radiator fan motor is finally turned "ON." Consequently, the idle speed is not made any more stable. Rather, the idle speed is just caused to fluctuate around a higher level which is intended to reduce the possibility of engine stall.
Accordingly, it would be highly desirable to provide a method of controlling the duty cycle of a PWM signal applied to the alternator field winding which better compensates for sudden, significant electrical loads experienced by the charging system during vehicle operation. An improved method of delivering the duty cycle would provide better control over the alternator output, which would help to prevent the abrupt changes in alternator loading on the engine which cause fluctuations in engine speed at low idle when various accessories of the vehicle are switched "ON" or "OFF." Such an improved method for controlling the duty cycle of a PWM signal applied to the alternator would also allow a more steady-state charging current to be applied to the battery by the alternator even when various electrical accessories of the vehicle are switched "ON" and "OFF."